1. Field of the Invention
The present application concerns a gyroscopic attitude indicator of the kind comprising a gyroscope suspended in a gimbal mount comprising inner and outer frames articulated one within the other about two orthogonal axes and means for sensing the spatial position of the axis of the gyroscope and for displaying this position in the form of an artificial horizon.
2. Description of the Prior Art
Longitudinal and lateral attitude indicators, usually called "artificial horizons", are well-known in aviation.
It is well known that when an aircraft begins to nose dive by rotating about its pitch axis the horizon, if the pilot can see it, tends to "rise" in his field of view. On the other hand, when the aircraft begins to pull up, the horizon tends to "fall" in the pilots field of view. Similarly, when the aircraft begins to roll, to the left for example, if the pilot can see the horizon it begins to rotate clockwise in his field of view while when the aircraft begins to roll to the right the horizon begins to rotate in the opposite direction.
It is also well known that when the aircraft begins a combined movement about its pitch and roll axes there results a combined movement of the horizon in the visual field of the pilot in the directions mentioned above.
The longitudinal and lateral attitude indicator is positioned on the instrument panel so as to be clearly visible to the pilot or pilots, its function being to display a dynamic image representing the horizon and the silhouette of the aircraft relative to the horizon as the aircraft performs simple or composite movements about its pitch and/or roll axis. Thus if the real horizon is not visible, because of the height at which the aircraft is flying or because of restricted or zero visibility, the pilot can tell the lateral and longitudinal attitude of the aircraft at a glance from the artificial horizon.
There are various families of methods available for creating a dynamic image showing in a schematic way the movement of an artificial horizon relative to the schematic representation of an aircraft. The method to which the present invention relates is one of those subsumed under the heading "gyroscopic artificial horizons".
Generally speaking, a gyroscopic artificial horizon comprises:
a gyroscope consisting for the main part of a mass rotated by a motor at very high speed (several thousand revolutions per minute), the axis of rotation of this mass being the axis of the gyroscope,
erector means adapted to maintain the axis of the gyroscope as close as possible to a vertical line through the point over which the aircraft carrying the gyroscopic artificial horizon is flying,
means for sensing the position of the gyroscope axis in space and means for displaying this position in the form of an artificial horizon.
In one embodiment of a gyroscopic artificial horizon known from French Pat. No. 1 142 614 and meeting the above definition the gyroscope is suspended in a gimbal mount comprising two frames articulated the one within the other about two orthogonal axes. The gyroscopic artificial horizon is mounted on the instrument panel of the aircraft in such a way that the rotation axis of the outer frame is parallel to the roll axis of the aircraft. The display means comprise a partially truncated sphere resembling in its overall shape a barrel. This "spherical barrel" is adapted to rotate about its longitudinal axis of symmetry and is mounted on an appropriate structure such that this longitudinal axis of symmetry is parallel to the rotation axis of the inner frame carrying the gyroscope. As a result, the rotation axis of the display barrel is perpendicular to the axis of the gyroscope and, by virtue of the erector means mentioned previously, when the gyroscope is operating the axis of rotation of the barrel remains perpendicular to the direction of the vertical line through the geographic point over which the aircraft is flying.
In this embodiment the gyroscopic artificial horizon comprises a toothed wheel constrained to rotate with the inner frame, about the axis about which the inner frame rotates relative to the outer frame. It meshes indirectly with a toothed wheel constrained to rotate with the display barrel around its longitudinal axis of symmetry so that when angular displacement of the inner frame relative to the outer frame occurs in one direction the display barrel rotates in the opposite direction through an angle which is the same as or proportional to this angular displacement.
All of this structure is mounted in a casing comprising on the front face a transparent screen at the centre of which is a schematic aircraft silhouette. An artificial horizon is schematically represented on the display spherical barrel by an arc of a great circle disposed in a plane containing the longitudinal axis of symmetry of the spherical barrel. The arrangement is such that when the aircraft is flying straight and level the schematic silhouette of the aircraft on the transparent face of the casing coincides with the artificial horizon.
In operation, as is well known, the artificial horizon takes up, in real time, a position relative to the schematic aircraft silhouette on the front of the casing which correpsonds substantially to the position of the aircraft in space.
Thus when the aircraft is in a neutral attitude relative to its roll axis the position of the silhoutte relative to the artificial horizon line provides an indication of the longitudinal attitude of the aircraft. Likewise, when the aircraft has a neutral attitude relative to its pitch axis, the position of the silhouette relative to the artificial horizon line represents the attitude of the aircraft relative to its roll axis. If the aircraft performs a combined motion about its pitch and roll axes the position of the silhouette relative to the artificial horizon line continues to represent the substantial position of the aircraft relative to the horizon.
This type of gyroscopic artificial horizon has been found extremely beneficial in use and in particular is serving as a back-up artificial horizon in a very large number of aircraft equipped for operation under instrument flight rules.
This artificial horizon can be used as a back-up instrument in the event of failure of the power supply to the onboard instrumentation. When the power supply to the gyroscope motor fails, because of the high inertia of its rotating mass the gyroscope continues to rotate at high speed until residual friction causes it to stop rotating. As a general rule, the artificial horizon will continue to function for between five and ten minutes, giving the pilot an indication of the aircraft's attitude during this time interval.
Although the reliability of these gyroscopic artificial horizons has been proven, research and development has continued in this area with a view to simplifying the mechanical structure of the gyroscope in order to enhance the reliability of the gyroscopic artificial horizon and to reduce its manufacturing cost.